Powder injection molded metal product and process

ABSTRACT

A powder injection molding process for making hard to make parts having internal cavities and hard to make parts made by the process. Metal powders having a variety of shapes and sizes are mixed with binder to make a feedstock. The feedstock is molded into at least two raw molded parts which are then sintered to join the parts together to form the hard to make part. This process eliminates all secondary operations in parts having internal cavities where core withdrawal is not possible. In preferred embodiment two raw parts are joined together during the sintering step without adding any materials to assist the bonding. This methodology can be used to make assemblies that have complex internal cavities such as filter sections and wave-guide components that are a low cost critical solution for the communications industry.

[0001] This invention relates to metal products with complicated internal cavities and to techniques for making them in particular when the metal is copper.

BACKGROUND OF THE INVENTION

[0002] Powder injection molding is a well-known process for making metal parts having complicated shapes. A metal is converted to fine powder. The powder is mixed with a binder to form a feedstock. The binder typically includes one or more binder materials that can be removed at temperatures lower than the melting point of the metal. Using an injection-molding machine the feedstock is injected into a mold cavity having the shape of the desired product except the shape is about 15 to 30 percent larger than the finished product. The molded parts are then placed in a solvent bath to dissolve the soluble components to make the molded material porous. The porous parts are next sintered to remove all of the remainder of the binder material leaving a part that is in the desired form and almost 100 percent metal. The sintering step typically reduces the size of the part by about 20 to 30 percent. A detailed description of a good powder injection molding process is included in U.S. Pat. No. 4,197,118, incorporated by reference herein.

[0003] Several prior art techniques for joining of two injection molded components parts are known. One known technique involves shrinking one injection molded component part around a second component part. The second component part may be fully dense or may be sintering at the same time. Conventionally, component parts have been joined by adding to the interface between the parts, an intermediate material such as a brazing material, solder, welding alloys, glues or epoxies.

SUMMARY OF THE INVENTION

[0004] The present invention provides a powder injection molding process for making hard to make parts having internal cavities and hard to make parts made by the process. Metal powders having a variety of shapes and sizes are mixed with solvent and binder to make a feedstock. The feedstock is molded into at least two raw molded parts, which are then sintered to join the parts together to form the hard to make part. This process eliminates all secondary operations in parts having internal cavities where core withdrawal is not possible. In preferred embodiment two raw parts are joined together during the sintering step without adding any materials to assist the bonding. This methodology can be used to make assemblies that have complex internal cavities such as filter sections and waveguide components that are a low cost critical solution for the communications industry. This methodology eliminates expensive secondary operations such as fixturing and eliminates any need for intermediate-bonding materials such as braze compounds, epoxies and glues. This invention also permits inspection of the critical internal features before the components are joined into the assembly. This technology can be used to make filter sections, wave-guide components and other parts with similar complex internal shapes for the communication and other industries at lower costs than other assembly methods. Assemblies can be made quickly and inexpensively with a very high yield comparable to that in the process for regular powder injection molded parts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIGS. 1A and 1B show two views of a part made pursuant to the present invention.

[0006]FIGS. 2A, 2B and 2C show three views of a mold pattern for the FIGS. 1A and 1B part.

[0007]FIGS. 3 through 7 show examples of hard to make parts.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0008] Hard to Make Part

[0009]FIG. 1A is a cross section view and FIG. 1B is a front view along the axis of a typical hard to make copper part (such as high frequency wave-guides or high frequency filters that are very difficult to fabricate with accurate dimensions. Attempts were made by Applicants to mold a part such as this as one piece using a copper powder technology. This effort was only partially successful. After repeated efforts by Applicants, precise dimensions on the inside surfaces could not be provided. The part could be made by die casting two aluminum halves, machining the halves to tolerance and then brazing, welding or gluing and mechanically fastening the two halves together. Applicants' have developed a better method of making parts like the part shown in FIGS. 1A and 1B.

[0010] Preferred Powdered Injection Molding Technique

[0011] Applicants have proven the feasibility of the present invention by making samples of a wave-guide filter section in high purity copper. The bond strength of the two joined components was demonstrated by a water pressure test at 100 psi without leakage or separation. These samples have been tested and found to meet the radio wave requirements.

[0012] Making the Metal Powder

[0013] In this preferred embodiment of the present invention, the metal powders used to make the components are pure copper to achieve the desired high electrical conductivity and are produced by water atomization to provide low cost and irregular particle shapes. The preferred water atomization process is described in detail in U.S. Pat. No. 4,080,126, issued Mar. 21, 1978. That patent is incorporated by reference herein. The powder is screened to minimize particles larger than 44 microns and to ensure sufficient fine particles in the powder lot size distribution so that the molded parts will sinter to sufficient density, typically at least 95% of full density, to provide strength and prevent leakage. The water atomization process produces very small irregularly shaped particles suitable for powder injection molding. Furthermore, Applicants have discovered, surprisingly, that these irregular shaped particles are important in effecting the joining of two separate component parts during sintering by providing interlocking pressure points at the joint interface, which enhances diffusion across the interface between the components being joined.

[0014] Making the Raw Parts

[0015] In this preferred embodiment parts such as the part shown in FIGS. 1A and 1B are made by injection molding two separate component parts and then joining the parts in a sintering process. To make the part shown in the figures, one mold cavity is prepared using an injection block and a top moving part to provide an enlarged cavity image of one half of the part shown in FIGS. 1A and 1B. The mold cavity is 28 percent larger than the finished part and each part comprises two small location pins (1.0 mm long and 1.0 mm in diameter) and two matching location pinholes having corresponding dimensions. FIG. 2A is a cross section of the mold with the mold closed showing the mold cavity 10, the mold runner 12 and the mold gate 14. FIG. 2B shows a side view of the moving part of the mold showing at 30 the location of the location pins and the location pin cavities. FIG. 2C shows a side view of the mold insert block with phantom outlines showing the location of the core forming elements in the moving part of the mold. Since the part is symmetrical, only one mold is needed. The powder is screened and tested to assure that the particle sizes are as desired. The powder is mixed with a binder material including soluble binding materials to produce the injection molding feedstock. The feedstock is then fed into a conventional injection-molding machine and raw component parts are produced in the conventional powder injection molding process. Applicants are able to produce as many as 60 to 180 raw parts per hour from one cavity using a Model 270S injection-molding machine available from Arburg, Inc.

[0016] Sintering and Joining in One Step

[0017] After the raw component parts are inspected a first part is placed on top of a second part. The top and bottom parts are processed through the solvent to remove the soluble binder components and then placed in an oven for sintering at a temperature slightly below the melting point of copper. The sintering step removes the remaining binder material, shrinks the parts to the desired size and permanently joins the top to the bottom part.

[0018] It is preferred that the components to be joined have interfaces that fit together as closely as possible in order to enhance the diffusion of copper between the two components. Some force perpendicular to the join interface is required. In many applications the weight of the top part applies sufficient force. Alternatively, a secondary inert weight can be used while some of the binder components are being removed in the solvent step and during sintering.

[0019] It is also preferred that shrinkage during sintering is at the maximum end of the spectrum normally used for powder injection molded parts. For example a scale-up multiplier from drawing to mold size in the order of 1.28× to 1.30× or more is preferred to aid bonding at the interface between component parts.

[0020] Initial sintering of the copper powders starts before the backbone binder component is burned off and during this time the structure of the components is weakest and the interface relaxes to accommodate small irregularities between the joining components. The sintering step encourages the diffusion of the copper atoms to not only fill spaces between the copper particles but also to cross the interface between components and thereby causing the components to become one assembled part. The preferred atmosphere in the furnace is a strong reducing gas to reduce oxides and any other surface films from the particle surfaces during sintering. Hydrogen gas is a preferred atmosphere for the copper powders. The heating profile in the furnace must follow a series of temperature rises and holds as for conventional metal powder injection molded components. This removes the backbone binder slowly and without breaking the components. A maximum sintering temperature is preferred to achieve the highest density and the strongest joint. For pure copper components there is a single melt point at 1083 C., which cannot be exceeded. A temperature about 2 to 10 C. below the melt point is a practical preferred maximum temperature. The preferred time at the maximum temperature for the water atomized copper powders is four hours.

FIRST PREFERRED EXAMPLE

[0021] 1. Feedstock Preparation

[0022] Two mixes of copper feedstock, identified as 2002-059 and 2002-060 were made using the following procedure. A 4,553-gram lot of water atomized pure copper powder was placed in a V-cone blender. The particle size distribution of the powder after blending was analyzed by Microtrac as follows:

[0023] 99.9% was less than 88.0 microns

[0024] 92.9% was less than 44.0 microns

[0025] 68.4% was less than 22.0 microns

[0026] 49.1% was less than 15.6 microns

[0027] 27.0% was less than 11.0 microns

[0028] 10.6% was less than 7.8 microns

[0029] 3.0% was less than 5.6 microns

[0030] Stearic acid, at 3% of the binder system, was added to the copper powder and all was blended together for 30 minutes.

[0031] The binder system consisted of 447 grams of the following constituents:

[0032] Polypropylene at 40%

[0033] Paraffin wax at 48%

[0034] Carnauba wax at 9%

[0035] Stearic acid at 3%

[0036] The binder components except for the Stearic acid were melted in a preheated sigma blade mixer. When the copper powders and Stearic acid had completed the blend cycle they were slowly added to the molten binders and the whole mixture was heated to 180 C. to ensure complete melting of all binder components. Then the temperature was lowered to 165 C. and held for one hour. Finally the mixer was evacuated with a rough vacuum pump for five minutes to pull out entrained air as the mix temperature was lowered to 155 C. After the five minute cooling period the mixer was stopped, the vacuum removed and the mix was unloaded from the mixer and cooled to room temperature. When cooled, the chunks of mix were granulated in a conventional granulator of the type used for regrinding plastic screws and runners. The resulting small pea sized particles are the feedstock for the molding machine.

[0037]2. Molding Parts

[0038] The copper feedstock was fed into a conventional injection-molding machine and injected into a mold cavity that was larger than the final component by 28%. The molded component contained two locating pins on one side and two matching pinholes on the other side. Two such parts could then be assembled together with the self-locating pins in the pinholes.

[0039] 3. Solvent Debinding

[0040] The molded copper assemblies were placed in a tank containing 1,1,1-Trichloroethane to remove the waxes and the Stearic acid from the binder system. The solvent was heated to 46 C. to shorten the binder removal time. The time to remove the soluble components was determined by weight loss from the as-molded parts and 14 hours was used. Other equivalent solvents also can be used to remove the soluble binder constituents.

[0041] 4. Sintering

[0042] The debound copper assemblies were removed from the solvent, dried and placed on ceramic furnace shelves. Additional ceramic setter pieces were placed under a raised section of the assembly for further support so it would not sag during sintering. The ceramic setter shelves were loaded into the furnace. The furnace was purged with nitrogen for 25 minutes to remove air and then hydrogen was introduced. The furnace was heated with a 100% hydrogen atmosphere in a series of ramps and holds as follows:

[0043] Ramp from room temperature to 490 C. in 70 minutes

[0044] Hold at 490 C. for 110 minutes

[0045] Ramp to 525 C. in 90 minutes

[0046] Hold at 525 C. for 90 minutes

[0047] Ramp to 590 C. in 90 minutes

[0048] Hold at 590 C. for 60 minutes

[0049] Ramp to 1081 C. in 130 minutes

[0050] Hold at 1081 C for 230 minutes

[0051] Furnace cool to room temperature

[0052] Importance of Water Atomized Copper

[0053] The concept of joining water atomized copper PIM components was conceived on Dec. 18, 2001. Although attempts at joining PIM parts in the sinter furnace had been tried previously with limited success, the use of PIM parts made from water atomized copper powders had not been tried in joining experiments. However, the authors thought that it might be possible to improve the join between two components by sintering the highly irregular copper powders across the interface between two components.

[0054] Applicant's Experiments

[0055] The first attempt to join two copper components was done by placing one separately molded part on top of another in the sinter furnace and sintering the assembly along with other production parts made of the same copper material. After sintering in the conventional production cycle it was found to Applicant's surprise that there was a very strong bond between the two components which could not be broken by striking one component with a hammer while the assembly was fixed in a vice. Subsequent sinter joining tests confirmed Applicants' surprising discovery. Next a special mold was built to make a component part that could be joined into one assembly using self-locating V-shaped grooves and ridges. However, it was found during sintering that one component would shift with respect to the other and ride up on the V-shaped grooves causing the joint to separate at various locations. Therefore, the mold was rebuilt without the V-grooves but used two pins and two pinholes to self-locate each component. Assemblies of these components with self-locating pins were sintered and found to make very uniform, strong leak-tight joins. It was also found that critical design features within the assembly cavity and measured across the join could be held to tolerances of approximately +/−0.001 in. which made the process practical for many types of assemblies.

[0056] Hard to Make Parts

[0057] Cross sections of several examples of hard to make parts are shown in FIGS. 3 through 7. Parts like these are easily and efficiently produced using the present invention. FIG. 3 shows a connector with a 90-degree bend. FIG. 4 shows a similar connector with a U bend. FIG. 5 shows an electrical connector housing for a sensor system. FIG. 6 shows an example of a part which is made hollow merely to reduce weight and/or save material costs. FIG. 7 shows a technique for making internal threads using a molding process.

[0058] While the present invention has been describer in terms of the above descriptions of preferred embodiments, the reader should understand that the present invention is not limited to those embodiments and that many modifications and variations of the above embodiments are possible without departing from the spirit of the invention. The scope of the invention should be determined by the appended claims and their legal equivalents. 

What is claimed is:
 1. A powder injection molding process for making hard-to-make parts having internal cavities comprising the following steps: A) metal powders having a variety of shapes and sizes are mixed with binder to make a feedstock, B) the feedstock is molded into at least two raw molded parts, C) the at least two molded parts are then sintered to join the parts together to form the hard-to-make part.
 2. A process as in claim 1 wherein said metal powders comprise copper.
 3. A process as in claim 1 wherein no material is added between the at least two parts to assist in the bonding.
 4. A process as in claim 1 wherein said at least two parts are two parts.
 5. A process as in claim 1 wherein said hard-to-make part is a wave guide component.
 6. A process as in claim 4 wherein said hard-to-make part is a filter section.
 7. A process as in claim 1 wherein said process is a part of a mass production process for making said hard-to-make parts.
 8. A process as in claim 1 wherein said raw molded parts taken together are about 15 percent to 35 percent larger than the hard-to-make part.
 9. A process as in claim 1 wherein said raw molded parts taken together are about 28 to 30 percent larger than the hard-to-make part.
 10. A process as in claim 1 wherein said metal powders of various sizes are prepared using a water atomization process.
 11. A process as in claim 1 wherein each of said raw molded parts comprise at least one location pin or pin hole for assuring proper positioning of the parts.
 12. A process as in claim 1 wherein a compressive force is applied to press the at least two parts together during the sintering process.
 13. A process as in claim 11 wherein one of the at least two parts is on top of another part and said compressive force is applied by adding a weight on top of the top part.
 14. A process as in claim 1 wherein said sintering takes place in a sintering furnace and a strong reducing gas is added to the sintering furnace during the sintering process.
 15. A process as in claim 13 wherein said strong reducing gas is hydrogen.
 16. A process as in claim 2 wherein said molded parts are sintered at temperatures about 2 to 10 degrees C. below a melt point for the metal powders.
 17. A process as in claim 16 wherein said powders are copper powders and said melt point is about 1083 C.
 18. A hard-to-make part having an internal cavity comprising: A) a first sintered part, B) a second sintered part solidly bound to said first sintered part at an interface forming said internal cavity with no added material at said interface.
 19. A hard-to-make part as in claim 18 wherein said hard-to-make part is a copper part.
 20. A hard-to-make part as in claim 18 wherein said hard-to-make part is a wave guide component.
 21. A hard-to-make part as in claim 20 wherein said hard-to-make part is a filter section. 